Just the opposite. Sure, DM was proposed when some observations didn't match predictions, but do you think scientists liked that? Do you think they just accepted it? They did not. No one liked DM, and many people tried to disprove it. How does one disprove DM? You simply assume that it exists, and then make a prediction using that assumption. If the prediction is wrong, that's evidence against DM. What happened is that every time someone made a DM prediction, it turned out to be correct.

Two early cases of this were that if invisible matter exists, there should be gravitational lensing around seemingly empty space, which was then found, and that if if something like dark matter existed before recombination, then a certain pattern should be found in the cosmic microwave background, and a few years after the prediction, sensitive enough equipment was deployed that could see enough detail in the CMB, and the prediction turned out correct.

Today there's something like a dozen such predictions that have come true, ranging in scale from kiloparsecs to gigaparsecs, from things happening today to things that happened in the early universe. When a theory makes accurate predictions across many problems and areas, that is good evidence for the theory.